Plant and process for producing a solid-state alkali metal cyanide product containing alkali metal hydroxide

Abstract

The present invention relates to a plant and to a process for producing a solid-state alkali metal cyanide product. In order to obtain a solid-state alkali metal cyanide product that has a good proportion by weight of alkali metal hydroxide, and in order to ensure that it remains as calm as possible in the plant during operation, it is proposed that the proportion by weight of alkali metal hydroxide in a suspension withdrawn from the crystallization vessel and sent to a solid-liquid separation device be increased before it enters the solid-liquid separation device.

Description

[0001] Plant and process for producing an alkali metal cyanide solid product containing alkali metal hydroxide

[0002] The present invention relates to a plant and a process for producing an alkali metal cyanide solid product containing alkali metal hydroxide.

[0003] In this case, the term "solid product" means that at least 50% by weight of the solid product is solid. The solid product may contain a liquid phase in addition to a solid phase.

[0004] When alkali metal cyanide comes into contact with an acid, it decomposes, releasing hydrogen cyanide. Contact with slightly carbonic water, which can occur, for example, when carbon dioxide from the ambient air dissolves in water, is sufficient.

[0005] To prevent the unintentional release of hydrogen cyanide during storage, handling, and use of alkali metal cyanide, alkali metal cyanide products contain a small amount of alkali metal hydroxide. The alkali metal hydroxide neutralizes the acid, e.g., carbonic acid, and thus prevents the decomposition of the alkali metal cyanide. On the other hand, too high a concentration of alkali metal hydroxide is problematic due to its hygroscopic effect, especially for rather dry alkali metal cyanide solid products, which can clump due to water absorbed from the air.

[0006] To produce alkali metal cyanide solid products containing alkali metal hydroxide, plants can be used with a crystallization tank into which aqueously dissolved alkali metal hydroxide is added, which was consumed during the formation of aqueously dissolved alkali metal cyanide. These plants may exhibit unstable operation with clearly audible knocking and pounding.

[0007] Such a system is known from GB 858,844 A. The system comprises an evaporative crystallizer having a crystallization vessel under reduced pressure with a suspension collection chamber. The suspension collection chamber contains a first suspension containing a liquid phase with aqueous sodium cyanide and aqueous sodium hydroxide dissolved in water, as well as a solid phase with sodium cyanide crystals. The first suspension from the suspension collection chamber is passed through a suspension withdrawal line into a rotary filter, which filters sodium cyanide crystals out of the first suspension. In this process, the first suspension is separated into a sodium cyanide solid product and a second suspension, which has a lower weight fraction of sodium cyanide crystals than the first suspension. The second suspension is passed into the suspension collection chamber of the crystallization vessel.Upstream of the rotary filter, a reaction loop branches off from the suspension removal line, in which a reactant gas, essentially pure gaseous hydrogen cyanide, is added to the first suspension. This creates a reaction mixture with a third suspension, which, compared to the first suspension, contains a higher proportion of aqueously dissolved sodium cyanide and a lower proportion of aqueously dissolved sodium hydroxide. The reaction mixture is fed into the suspension collection chamber of the crystallization vessel.

[0008] Vapors rise from the suspension collection chamber. If the reaction mixture contains a gas phase, this separates and enriches the vapors.

[0009] The vapors undergo a gas scrubbing process. A sieve tray is provided in the upper section of the crystallization vessel, which is filled with an aqueous sodium hydroxide solution, from which sodium hydroxide solution rains down. This rain is encountered by the vapors rising to the sieve tray. The vapors must also flow through the sieve tray. If the vapors contain hydrogen cyanide, further aqueous sodium cyanide is formed upon contact with the sodium hydroxide solution. The remaining sodium hydroxide in the rain is used to replenish the sodium hydroxide reacted in the reaction loop.

[0010] GB 858,844 A states that a sodium hydroxide concentration of more than approximately 4% in the mother liquor in the suspension collection chamber leads, on the one hand, to severe rumbling and pounding, as well as to strong vibrations in the circulation system, and, on the other hand, to an undesirable product with an excessively high sodium hydroxide content. Therefore, it is proposed to keep the sodium hydroxide concentration of the mother liquor in the suspension collection chamber in a range of 0.2 to 3%, preferably below 1%.

[0011] DE 1064934 A describes that when using strong alkali hydroxide, the reaction with hydrogen cyanide can begin very violently. When the hydrogen cyanide enters the alkali hydroxide, shocks can occur due to the heating of the reaction mixture by the released reaction heat. To solve this problem, it is proposed to use an alkali hydroxide solution already containing alkali cyanide and to set the liquid in the reaction vessel into such a vigorous rotation that a noticeable vortex forms.

[0012] Even from a suspension with a comparatively low alkali metal hydroxide concentration, a comparatively dry alkali metal cyanide solid product with a comparatively high alkali metal hydroxide concentration can be produced. However, this requires a correspondingly higher residual moisture content to remain in the solid product during solid-liquid separation, which then requires further drying. However, a higher residual moisture content has several disadvantages. For example, more energy is required for further drying than if the separation had been carried out with lower residual moisture. The more intensive the drying process, the more the decomposition of alkali metal cyanide is promoted, forming undesirable alkali metal formate and ammonia. A higher residual moisture content is also associated with a greater tendency for caking.In a manufacturing plant, caking and build-up can lead to problems, particularly blockages and poor heat transfer to heating and cooling surfaces.

[0013] US 4,847,062 A discloses, in Figure 3, a plant and a method for producing a sodium cyanide solid product. The plant comprises a vessel in which absorption and crystallization take place and in which a first suspension is produced. The first suspension contains a liquid phase with aqueously dissolved sodium cyanide and aqueously dissolved sodium hydroxide, as well as a solid phase with sodium cyanide crystals. The first suspension is withdrawn from the vessel through a suspension withdrawal line. A portion of it is fed into a recirculation loop. The remainder is fed into a solid-liquid separation device, in which the introduced first suspension is separated into a sodium cyanide solid product and a second suspension. The second suspension has a lower weight fraction of sodium cyanide crystals than the first suspension.The second suspension is fed into the recycle loop and mixed with the first suspension, creating a third suspension. A sodium hydroxide solution is added to the third suspension, creating a fourth suspension. The fourth suspension is introduced into the top of the vessel. Synthesis gas containing hydrogen cyanide is introduced into the bottom of the vessel.

[0014] DE 1235873 A describes a plant for producing an alkali metal cyanide solid, which comprises a receiver containing a first suspension. The first suspension has a liquid phase with aqueously dissolved alkali metal cyanide and aqueously dissolved alkali metal hydroxide, as well as a solid phase with alkali metal cyanide crystals. The first suspension is removed from the receiver and passed into a filter system in which alkali metal cyanide crystals are separated. This produces a second suspension with a lower weight fraction of alkali metal cyanide crystals than the first suspension. An alkali metal hydroxide solution is added to the second suspension, forming a third suspension. The third suspension is passed into an absorption tube, down the inner wall of which it flows. At the same time, a gas mixture containing hydrogen cyanide is introduced into the absorption tube.At the lower end of the absorption tube, a fourth suspension and a reaction gas mixture are present, flowing into the receiver. The reaction gas mixture fills an upper section of the receiver and flows out through a line.

[0015] US 3,079,239 A discloses a system and a method for the batchwise production of an aqueous sodium cyanide solution. The system comprises a reaction vessel in which a first sodium cyanide solution containing aqueously dissolved sodium cyanide and aqueously dissolved sodium hydroxide is produced. An aqueous sodium hydroxide solution and liquid hydrogen cyanide are introduced into the vessel. The supply of liquid hydrogen cyanide is regulated depending on the sodium hydroxide weight fraction of the first sodium cyanide solution. To determine the sodium hydroxide weight fraction of the first sodium cyanide solution, the first sodium cyanide solution is fed from the reaction vessel into a measuring loop, where pure water is added for dilution, producing a second sodium cyanide solution. The second sodium cyanide solution is fed into a measuring unit, which measures the electrical conductivity of the second sodium cyanide solution.From this, the sodium hydroxide weight fraction of the first sodium cyanide solution is determined. The second sodium cyanide solution is introduced into the reaction vessel.

[0016] The object of the present invention is to provide a plant with a crystallization vessel with which an alkali metal cyanide solid product can be produced which has a good weight fraction of alkali metal hydroxide, while remaining as quiet as possible in the plant.

[0017] The object is achieved according to the invention with a system having the features of claim 1. In the system according to the invention, the liquor admixing device is arranged downstream of the crystallization tank and upstream of the solid-liquid separation device. The liquor admixing device can be used to add the reactant liquid containing aqueously dissolved alkali metal hydroxide to the first suspension. This allows a suspension (second suspension) to be fed to the solid-liquid separation device whose liquid phase has a higher alkali metal hydroxide weight fraction than the liquid phase of the suspension located in the suspension collection chamber of the crystallization tank (first suspension).In this way, an alkali metal cyanide solid product can be obtained which, in order to provide greater safety against the unintentional release of hydrogen cyanide, has a higher alkali metal hydroxide weight fraction than an alkali metal cyanide solid product which would otherwise be obtained by solid-liquid separation of the suspension located in the suspension collection space of the crystallization vessel (first suspension).At the same time, the alkali metal hydroxide weight fraction of the liquid phase of the suspension located in the suspension collection space of the crystallization vessel (first suspension) can be selected to be lower in order to ensure a quieter process in the plant than would be possible if the suspension located in the suspension collection space of the crystallization vessel (first suspension) already had to have the entire alkali metal hydroxide content required for the desired alkali metal cyanide solid product with greater security against unwanted release of hydrogen cyanide.

[0018] Preferably, water from the liquid phase of the first suspension can be evaporated in the crystallization vessel. This promotes the formation and growth of alkali metal cyanide crystals. Advantageously, the second separation device discharge line can fluidically connect the solid-liquid separation device and the crystallization vessel of the suspension production device, and the third suspension can be discharged from the solid-liquid separation device through the second separation device discharge line and fed into the crystallization vessel. Thus, the third suspension can be used to produce further alkali metal cyanide solid product. In this process, alkali metal hydroxide, which was consumed in the formation of alkali metal cyanide, can also be refilled into the crystallization vessel with the third suspension.Less alkali metal hydroxide enters the crystallization vessel with the third suspension than if the reactant liquid containing aqueous alkali metal hydroxide were introduced directly into the crystallization vessel instead of being added to the first suspension in the caustic solution mixing device. Because the liquid phase of the second suspension has a higher weight fraction of alkali metal hydroxide than the liquid phase of the first suspension, more alkali metal hydroxide enters the alkali metal cyanide solid product. Consequently, this excess alkali metal hydroxide can no longer enter the crystallization vessel.

[0019] Advantageously, the second separation device discharge line can have a pressure drop limiting device through which the third suspension can flow and with which an operating pressure difference can be maintained between the solid-liquid separation device and the crystallization vessel of the suspension production device while the third suspension flows through the pressure drop limiting device. Thus, the flow of the third suspension from the solid-liquid separation device into the crystallization vessel can be pressure-neutral for the operating pressure in the crystallization vessel.

[0020] Advantageously, a reaction loop can be provided for producing a fourth suspension which contains a liquid phase with aqueously dissolved alkali metal cyanide and aqueously dissolved alkali metal hydroxide and a solid phase with alkali metal cyanide crystals, wherein

[0021] • first suspension can be introduced into the reaction loop,

[0022] • the reaction loop has a hydrogen cyanide admixing device with which the fourth suspension, which has a lower weight fraction of aqueously dissolved alkali metal hydroxide and a higher weight fraction of alkali metal cyanide compared to the first suspension, can be produced by admixing a liquid reactant liquid containing hydrogen cyanide to the first suspension, and

[0023] • fourth suspension can be introduced into the crystallization vessel.

[0024] This allows the formation of alkali metal cyanide to occur outside the crystallization vessel, which further contributes to quiet processes.

[0025] Preferably, • the reaction loop can be fluidically connected to the suspension removal line, wherein first suspension can be removed from the suspension removal line and fed into the reaction loop, and

[0026] • the reaction loop must be fluidically connected to the crystallization vessel and the fourth suspension must be fed from the reaction loop into the crystallization vessel.

[0027] In this way, the reaction loop can be fed from the suspension withdrawal line and the suspension collection chamber only needs to be provided with one outlet for discharging the first suspension instead of two.

[0028] Particularly preferably, a lye supply line can be provided which is fluidly connected to the crystallization tank and through which a reactant liquid containing aqueously dissolved alkali metal hydroxide can be fed into the crystallization tank. This provides a further possibility for introducing aqueously dissolved alkali metal hydroxide into the crystallization tank, in particular to replenish alkali metal hydroxide consumed during the formation of alkali metal cyanide. Furthermore, this allows for a smaller amount of alkali metal hydroxide added to the first suspension via the lye admixing device.

[0029] Particularly advantageously, the liquor mixing device can be fluidly connected to the liquor supply line, and reactant liquid containing aqueously dissolved alkali metal hydroxide can be fed from the liquor supply line into the liquor mixing device. Thus, both the liquor mixing device and the crystallization tank can be fed from a single source of reactant liquid containing aqueously dissolved alkali metal hydroxide.

[0030] It is a further object of the present invention to provide a process by which an alkali metal cyanide solid product can be produced which has a good weight fraction of alkali metal hydroxide, while remaining as quiet as possible in a plant used therein.

[0031] This object is achieved according to the invention with a method having the features of claim 9. By mixing the first suspension with the reactant liquid containing aqueously dissolved alkali metal hydroxide, a suspension (second suspension) can be fed to the solid-liquid separation device whose liquid phase has a higher alkali metal hydroxide weight fraction than the liquid phase of the suspension located in the suspension collection space of the crystallization vessel (first suspension). In this way, an alkali metal cyanide solid product can be obtained which, in favor of greater safety against the unintentional release of hydrogen cyanide, has a higher alkali metal hydroxide weight fraction than an alkali metal cyanide solid product that would otherwise be obtained by solid-liquid separation of the suspension located in the suspension collection space of the crystallization vessel (first suspension).At the same time, the alkali metal hydroxide weight fraction of the liquid phase of the suspension located in the suspension collection space of the crystallization vessel (first suspension) can be selected to be lower in order to ensure a quieter process in the plant than would be possible if the suspension located in the suspension collection space of the crystallization vessel (first suspension) already had to have the entire alkali metal hydroxide content required for the desired alkali metal cyanide solid product with greater security against unwanted release of hydrogen cyanide.

[0032] Preferably, a third suspension from the solid-liquid separation device can be fed into the crystallization vessel of the suspension production device. This allows the third suspension to be used to produce additional alkali metal cyanide solid product. In this process, the third suspension can also be used to replenish the crystallization vessel with alkali metal hydroxide consumed in the formation of aqueously dissolved alkali metal cyanide. Less alkali metal hydroxide enters the crystallization vessel with the third suspension than if the reactant liquid containing aqueously dissolved alkali metal hydroxide were introduced directly into the crystallization vessel instead of being added to the first suspension in the liquor admixture device.Because the liquid phase of the second suspension has a higher weight fraction of alkali metal hydroxide than the liquid phase of the first suspension, more alkali metal hydroxide enters the alkali metal cyanide solid product. Consequently, this excess alkali metal hydroxide can no longer enter the crystallization vessel.

[0033] Advantageously, an operating pressure difference can be maintained between the solid-liquid separation device and the crystallization vessel while the third suspension is fed from the solid-liquid separation device into the crystallization vessel. In this way, the third suspension can flow from the solid-liquid separation device into the crystallization vessel in a pressure-neutral manner for the operating pressure in the crystallization vessel.

[0034] Particularly preferred is

[0035] • first suspension is fed into a reaction loop,

[0036] • in the reaction loop, a fourth suspension is prepared by admixing a starting liquid containing liquid hydrogen cyanide to the first suspension, which fourth suspension contains a liquid phase with aqueously dissolved alkali metal cyanide and aqueously dissolved alkali metal hydroxide and a solid phase with alkali metal cyanide crystals, wherein the fourth suspension has a lower weight fraction of aqueously dissolved alkali metal hydroxide and a higher weight fraction of alkali metal cyanide compared to the first suspension, and

[0037] • the fourth suspension is fed into the crystallization vessel. This allows the formation of aqueously dissolved alkali metal cyanide to take place outside the crystallization vessel, which further contributes to smooth processes. Preferably, a reactant liquid containing aqueously dissolved alkali metal hydroxide can be fed into the crystallization vessel through a lye feed line. This provides a further option for introducing aqueously dissolved alkali metal hydroxide into the crystallization vessel, in particular to replenish alkali metal hydroxide consumed during the formation of alkali metal cyanide. Furthermore, the amount of alkali metal hydroxide added to the first suspension to form the second suspension can be selected to be smaller.

[0038] Particularly advantageously, reactant liquid containing aqueously dissolved alkali metal hydroxide can be removed from the lye feed line and mixed into the first suspension in step (c).

[0039] In this way, both the reactant liquid used in step (c) and the reactant liquid which is fed (directly) into the crystallization vessel can be supplied from one source.

[0040] Figure 1 shows an embodiment of the invention using a schematic flow diagram and is described below.

[0041] Figure 1 illustrates, using a schematic flow diagram, a plant 1 for producing an alkali metal cyanide solid product containing alkali metal hydroxide. The plant 1 comprises a suspension production device 2 with a crystallization vessel 3, inside which a suspension collection chamber 5 is provided for collecting the suspension. The suspension collection chamber 5 contains a first suspension, which contains a liquid phase with aqueously dissolved alkali metal hydroxide and aqueously dissolved alkali metal cyanide, as well as a solid phase with alkali metal cyanide crystals. The first suspension has a surface 6, which is also identified in Figure 1 by a surface symbol 7.

[0042] In the region of a lower end 8 of the crystallization vessel 3, the crystallization vessel 3 has a first outlet 9, to which a suspension removal line 10 is fluidly connected. The first suspension can be removed from the suspension collection chamber 5 through the suspension removal line 10. The suspension removal line 10 has a first pump 11.

[0043] The system 1 has a liquor admixing device 12, with which a reactant liquid containing aqueously dissolved alkali metal hydroxide can be admixed to the first suspension to produce a second suspension containing a liquid phase with aqueously dissolved alkali metal cyanide and aqueously dissolved alkali metal hydroxide, as well as a solid phase with alkali metal cyanide crystals. The reactant liquid containing aqueously dissolved alkali metal hydroxide, the alkali metal hydroxide weight fraction of which is higher than the alkali metal hydroxide weight fraction of the liquid phase of the first suspension, can be admixed. The liquor admixing device 12 has a first inlet 13, a second inlet 14, and an outlet 15. The suspension removal line 10 is fluidly connected to the first inlet 13. The suspension removal line 10 thus fluidly connects the suspension collection chamber 5 of the crystallization tank 3 and the liquor admixing device 12.

[0044] The first suspension from the suspension collection chamber 5 can be fed into the lye mixing device 12 through the suspension removal line 10. A first lye supply line 16 is fluidly connected to the second inlet 14. The reactant liquid containing aqueously dissolved alkali metal hydroxide can be fed into the lye mixing device 12 through the first lye supply line 16.

[0045] A caustic soda mixing device discharge line 17 is fluidly connected to the outlet 15. Second suspension from the caustic soda mixing device 12 can be discharged through the caustic soda mixing device discharge line 17.

[0046] Plant 1 comprises a solid-liquid separation device 18, with which, as part of a separation of the solid phase of the second suspension, the second suspension can be separated into an alkali metal cyanide solid product and a third suspension. The third suspension contains a liquid phase with aqueously dissolved alkali metal hydroxide and aqueously dissolved alkali metal cyanide, as well as a solid phase with alkali metal cyanide crystals. Compared to the second suspension, the alkali metal cyanide crystal weight fraction of the third suspension is lower.

[0047] The solid-liquid separation device 18 has an inlet 19, a first outlet 20, and a second outlet 21. The lye admixing device discharge line 17 is fluidly connected to the inlet 19. The lye admixing device discharge line 17 thus fluidly connects the lye admixing device 12 and the solid-liquid separation device 18. A second suspension can be introduced into the solid-liquid separation device 18 through the lye admixing device discharge line 17.

[0048] A first separator discharge line 22 is fluidically connected to the first outlet 20. Alkali metal cyanide solid product from the solid-liquid separation device 18 can be discharged through the first separator discharge line 22.

[0049] A second separator discharge line 23 is fluidically connected to the second outlet 21. A third suspension can be discharged from the solid-liquid separation device 18 through the second separator discharge line 23.

[0050] The second separator discharge line 23 is fluidically connected to a first inlet 24 of the crystallization tank 3. This means that the second separator discharge line 23 fluidically connects the solid-liquid separation device 18 and the crystallization tank 3. Through the second separator discharge line 23, a third suspension can be discharged from the solid-liquid separation device 18 and introduced into the crystallization tank 3. The first inlet 24 of the crystallization tank 3 is provided at a location below the liquid level 6 of the first suspension located in the suspension collection chamber 5 of the crystallization tank 3.

[0051] The second separation device discharge line 23 has a pressure drop limiting device 25 through which the third suspension can flow, allowing an operating pressure difference to be maintained between the interior of the solid-liquid separation device 18 and the interior 4 of the crystallization vessel while the third suspension flows through the pressure drop limiting device 25. The flow of the third suspension from the solid-liquid separation device 18 into the crystallization vessel 3 can therefore be pressure-neutral for the operating pressure in the crystallization vessel 3.

[0052] The pressure drop limiting device 25 can preferably comprise a fluid lock. To implement the fluid lock function, a U-shaped line section operating according to the principle of a siphon can be provided, for example. Such a fluid lock could also be referred to as a siphon-type fluid lock. This type of pressure drop limiting device 25 can be designed as a so-called feed well.

[0053] The suspension production device 2 has a reaction loop 26 with which a fourth suspension can be produced which contains a liquid phase with aqueously dissolved alkali metal hydroxide and aqueously dissolved alkali metal cyanide and a solid phase with alkali metal cyanide crystals, wherein the fourth suspension has a lower weight fraction of alkali metal hydroxide and a higher weight fraction of alkali metal cyanide than the first suspension.

[0054] The reaction loop 26 has a hydrogen cyanide admixing device 27, with which a reactant liquid containing hydrogen cyanide can be admixed to the first suspension. The hydrogen cyanide admixing device 27 has a first inlet 28, a second inlet 29, and an outlet 30. A reaction feed line 31 is fluidly connected to the first inlet 28. In the present embodiment of the invention, the reaction feed line 31 is also fluidly connected to the suspension removal line 10. It thus fluidly connects the suspension removal line 10 to the hydrogen cyanide admixing device 27. The first suspension can be fed from the suspension removal line 10 into the hydrogen cyanide admixing device 27 through the reaction feed line 31. The reaction feed line 31 has a second pump 32.

[0055] A hydrogen cyanide supply line 33 is fluidly connected to the second inlet 29. The reactant liquid containing liquid hydrogen cyanide can be introduced into the hydrogen cyanide admixing device through the hydrogen cyanide supply line 33.

[0056] A reaction discharge line 34 is fluidically connected to the outlet 30 of the hydrogen cyanide admixing device 27. A fourth suspension can be discharged from the hydrogen cyanide admixing device 27 through the reaction discharge line 34. The reaction discharge line 34 is also fluidically connected to a second inlet 35 of the crystallization vessel 3. It thus fluidically connects the hydrogen cyanide admixing device 27 and the crystallization vessel 3. A fourth suspension can be conveyed into the crystallization vessel 3 through the reaction discharge line 34.

[0057] The second inlet 35 of the crystallization tank 3 is provided at a location below the liquid level 6 of the first suspension located in the suspension collecting space 5 of the crystallization tank 3.

[0058] The reaction discharge line 34 has a heat exchanger 36 with which heat can be supplied to the fourth suspension.

[0059] The system 1 has a second liquor supply line 37, which is fluidically connected to a third inlet 38 of the crystallization tank 3. A reactant liquid containing aqueously dissolved alkali metal hydroxide can be introduced into the crystallization tank 3 through the liquor supply line 37 using the third inlet 38. The third inlet 38 is provided at a point above the liquid level 6 of the first suspension located in the suspension collection chamber 5 of the crystallization tank 3. With the aid of a distribution device (not shown in Figure 1), the reactant liquid is distributed over a peripheral region of the crystallization tank 3 and directed to an inner side 39 of a wall 40 of the crystallization tank 3, where it then runs down the inner side 39. Where the reactant liquid runs down the inner side 39, it counteracts alkali metal cyanide deposits on the inner side 39.

[0060] In the present embodiment of the invention, the first lye supply line 16 is fluidically connected to the second lye supply line 37 and is fed by the latter with the reactant liquid containing aqueously dissolved alkali metal hydroxide.

[0061] A gas scrubbing module 41 is provided above the distribution device in the interior 4 of the crystallization vessel 3. The space between the suspension collection chamber 10 and the gas scrubbing module 41 is a first gas chamber 42, and the space above the gas scrubbing module 41 is a second gas chamber 43. The gas scrubbing module 41 can be used to scrub hydrogen cyanide contained in vapors rising upward through the first gas chamber 42. This allows gas scrubbing to take place.

[0062] The crystallization vessel 3 has a fourth inlet 44 for feeding the gas scrubbing module 41 with a gas scrubbing liquid containing aqueously dissolved alkali metal hydroxide, said fourth inlet being provided at the level of an upper region 45 of the gas scrubbing module 41, and a second outlet 46 for removing gas scrubbing liquid from the gas scrubbing module 41, said second outlet 46 being provided at the level of a lower region 47 of the gas scrubbing module 41. A scrubbing module supply line 48 for supplying gas scrubbing liquid is fluidically connected to the fourth inlet 44, and a scrubbing module discharge line 49 for removing gas scrubbing liquid is fluidically connected to the second outlet 46. The removed gas scrubbing liquid remains separated from the suspensions and the solid product. It therefore flows neither into any of the first to fourth suspensions nor into the solid product.

[0063] In the region of an upper end 50 of the crystallization vessel 3, a third outlet 51 is provided, to which a vapor extraction line 52 is fluidly connected. Vapors that have passed through the gas scrubbing module 41 can be removed from the second gas chamber 43 of the crystallization vessel 3 through the vapor extraction line 52.

[0064] The system 1 has a vacuum unit 53 with an inlet 54, a first outlet 55, and a second outlet 56. The vapor extraction line 52 fluidly connects the second gas chamber 43 of the crystallization vessel 3 to the inlet 54 of the vacuum unit 53. The vacuum unit 53 can be used to suck in the vapors and generate the negative pressure required for the operation of the crystallization vessel 3.

[0065] Condensate forming in the vacuum unit 53 can be discharged from the vacuum unit 53 through a first vacuum unit discharge line 57 fluidically connected to the first outlet 55 of the vacuum unit 53. The remaining material flow can be discharged from the vacuum unit 53 through a second vacuum unit discharge line 58 fluidically connected to the second outlet 56 of the vacuum unit 53.

[0066] In the production process carried out using system 1, a first suspension is withdrawn from the suspension withdrawal line 10 and fed into the reaction loop 26. In the present embodiment of the production process, a reactant liquid containing hydrogen cyanide is admixed to the first suspension in the hydrogen cyanide admixing device 27. Alkali metal hydroxide from the liquid phase of the first suspension and hydrogen cyanide from the reactant liquid are converted to alkali metal cyanide and water. Accordingly, the fourth suspension has a lower weight fraction of alkali metal hydroxide and a higher weight fraction of alkali metal cyanide than the first suspension.

[0067] New alkali metal cyanide crystals form and existing alkali metal cyanide crystals grow when the saturation limit for alkali metal cyanide is reached in the liquid phase and further alkali metal cyanide is formed in the liquid phase and / or water is removed from the liquid phase, for example by evaporation. In the present embodiment of the production process, both occur. In the reaction loop 26, alkali metal cyanide is formed in the liquid phase, and water is evaporated in the crystallization vessel 3. In order to be able to evaporate the water at low temperatures, the crystallization vessel 3 is operated under reduced pressure. The heat required for the evaporation of water originates in part from the exothermic reaction of alkali metal hydroxide and hydrogen cyanide to form alkali metal cyanide and water. A further portion of heat is introduced by further heating the fourth suspension with the aid of the heat exchanger 36.The heat exchanger 36 thus functions as a reboiler. For example, the heat exchanger 36 can heat the fourth suspension to a temperature of approximately 65 °C.

[0068] When water evaporates, alkali metal cyanide may decompose, releasing hydrogen cyanide. This means that vapors containing hydrogen cyanide may be produced in crystallization vessel 3.

[0069] The vapors flow through the gas scrubbing unit 41, where hydrogen cyanide is scrubbed from the vapors. The now purified vapors flow into the second gas chamber 43 and leave the crystallization vessel 3 through the outlet 51. They are discharged through the vapor extraction line 52. The conveying action of the vacuum unit 53—which draws the vapors from the second gas chamber 43 through the vapor extraction line 52—maintains the desired operating negative pressure in the crystallization vessel 3.

[0070] The portion of the first suspension that flows past the connection point of the reaction loop 26 and continues through the suspension withdrawal line 10 is admixed in the lye admixing device 12, i.e., downstream of the connection point of the reaction loop 26, with a reactant liquid containing aqueously dissolved alkali metal hydroxide, the weight fraction of which is higher than that of the liquid phase of the first suspension. This produces the second suspension, the liquid phase of which has a higher weight fraction of alkali metal hydroxide than the liquid phase of the first suspension.

[0071] The reactant liquid containing aqueously dissolved alkali metal hydroxide is fed to the lye mixing device 12 through the first lye feed line 16.

[0072] The second suspension is discharged from the caustic admixing device 12 through the caustic admixing device discharge line 17 and introduced into the solid-liquid separation device 18. In the solid-liquid separation device 18, the second suspension is separated into an alkali metal cyanide solid product and a third suspension as part of the separation of the solid phase of the second suspension. The third suspension has a lower weight fraction of alkali metal cyanide crystals than the second suspension.

[0073] The alkali metal cyanide solid product is discharged from the solid-liquid separator 18 through the first separator discharge line 22 and the third suspension through the second separator discharge line 23.

[0074] The solid-liquid separation device 18 is a type of solid-liquid separation device that does not completely separate the liquid phase from the solid phase. It can be designed, for example, as a filter or centrifuge. Since the solid-liquid separation device 18 does not completely separate the liquid phase from the solid phase, the alkali metal cyanide solid product also contains a liquid phase. The weight fraction of this liquid phase depends in particular on the effectiveness with which the solid phase is separated from the liquid phase in the solid-liquid separation device 18.

[0075] The alkali metal cyanide solid product can, for example, be a crystal slurry or a merely wet or moist crystal product. If an even drier alkali metal cyanide solid product is desired, the alkali metal cyanide solid product discharged from the solid-liquid separation device 18 can be subjected to further drying.

[0076] Because the liquid phase of the second suspension has a higher weight fraction of alkali metal hydroxide than that of the first suspension, the alkali metal cyanide solid product has a higher weight fraction of alkali metal hydroxide than if the first suspension had been introduced into the solid-liquid separation device 18 instead of the second suspension.

[0077] The third suspension is passed through the second separator discharge line 23 into the crystallization vessel 3. Since the liquid phase of the third suspension has a higher weight fraction of alkali metal hydroxide than that of the first suspension, introducing the third suspension into the crystallization vessel 23 replenishes a further portion of the alkali metal hydroxide consumed in the reaction loop 26 during the formation of alkali metal cyanide.

[0078] The absolute operating pressure in the solid-liquid separation device 18 is higher than in the crystallization tank 3. The crystallization tank 3 is operated under negative pressure, which lowers the boiling point of water in the crystallization tank 3. The pressure drop limiting device 25 of the second separation device discharge line 23 ensures that the flow of the third suspension from the solid-liquid separation device 18 into the crystallization tank 3 is pressure-neutral for the operating pressure in the crystallization tank 3.

[0079] The alkali metal hydroxide used may, for example, be sodium hydroxide, in which case the resulting alkali metal cyanide solid product would be a sodium cyanide solid product, or potassium hydroxide, in which case the resulting alkali metal cyanide solid product would be a potassium cyanide solid product.

Claims

Patent claims 1. Plant (1) for producing an alkali metal hydroxide-containing alkali metal cyanide solid product, wherein the plant (1) comprises: (i) a suspension preparation apparatus (2) for preparing a first suspension containing a liquid phase with aqueous alkali metal cyanide and aqueous alkali metal hydroxide, and a solid phase containing alkali metal cyanide crystals, wherein the suspension preparation apparatus (2) comprises a crystallization vessel (3) in which a suspension collection chamber (5) is provided for collecting the first suspension; (ii) a suspension withdrawal line (10) for removing the first suspension from the suspension collection chamber (5) of the crystallization vessel (3); (iii) an alkali mixing device (12) for producing a second suspension, which contains a liquid phase with aqueous alkali metal cyanide and aqueous alkali metal hydroxide, and a solid phase with alkali metal cyanide crystals, by mixing a reactant liquid containing aqueous alkali metal hydroxide, the alkali metal hydroxide weight fraction of which is higher than the alkali metal hydroxide weight fraction of the liquid phase of the first suspension, to the first suspension, wherein the suspension extraction line (10) fluidically connects the suspension collection chamber (5) of the crystallization vessel (3) of the suspension production device (2) and the alkali mixing device (12), and the suspension extraction line (10) allows first suspension to be drawn from the suspension collection chamber (5) into the alkali mixing device (12); (iv) a lye mixing device discharge line (17) for discharging second suspension from the lye mixing device (12); (v) a solid-liquid separation device (18) with which the second suspension can be separated by separating the solid phase of the second suspension into an alkali metal cyanide solid product and a third suspension, which contains a liquid phase with aqueous alkali metal cyanide and aqueous alkali metal hydroxide, as well as a solid phase with alkali metal cyanide crystals, wherein the alkali mixing device discharge line (17) fluidically connects the alkali mixing device (12) and the solid-liquid separation device (18) and discharges the second suspension from the The alkali mixing device (12) can be directed into the solid-liquid separation device (18); (vi) a first separation device discharge line (22) for the discharge of alkali metal cyanide solid product from the solid-liquid separation device (18); (vii) a second separation device discharge line (23) for discharging a third suspension from the solid-liquid separation device (18).

2. System according to claim 1, characterized in that water of the liquid phase of the first suspension can be evaporated in the crystallization vessel (3).

3. System according to claim 1 or 2, characterized in that the second separation device discharge line (23) fluidically connects the solid-liquid separation device (18) and the crystallization container (3) of the suspension production device (2) and third suspension can be discharged from the solid-liquid separation device (18) and directed into the crystallization container (3) through the second separation device discharge line (23).

4. Plant according to claim 3, characterized in that the second separation device discharge line (23) has a pressure drop limiting device (25) which can be flowed through by third suspension and with which an operating pressure difference between the solid-liquid separation device (18) and the crystallization vessel (3) of the suspension production device (2) can be maintained while the pressure drop limiting device (25) is flowed through by third suspension.

5. Plant according to one of the preceding claims, characterized in that a reaction loop (26) is provided for producing a fourth suspension, which contains a liquid phase with aqueous alkali metal cyanide and aqueous alkali metal hydroxide, and a solid phase with alkali metal cyanide crystals, wherein • the first suspension can be introduced into the reaction loop (26), • the reaction loop (26) has a hydrogen cyanide mixing device (27) with which the fourth suspension, which has a lower weight fraction of aqueous dissolved alkali metal hydroxide and a higher weight fraction of alkali metal cyanide compared to the first suspension, can be produced by adding a liquid reactant liquid containing hydrogen cyanide to the first suspension, and • fourth suspension can be introduced into the crystallization vessel (3).

6. System according to claim 5, characterized in that • the reaction loop (26) is fluidically connected to the suspension extraction line (10), wherein first suspension can be drawn off from the suspension extraction line (10) and directed into the reaction loop (26), and • the reaction loop (26) is fluidically connected to the crystallization vessel (3) and fourth suspension can be directed from the reaction loop (26) into the crystallization vessel (3). Apparatus according to one of the preceding claims, characterized in that a caustic feed line (37) is provided which is fluidically connected to the crystallization vessel (3) and through which an aqueous solution of alkali metal hydroxide-containing reactant liquid can be directed into the crystallization vessel (3). Apparatus according to claim 7, characterized in that the caustic mixing device (12) is fluidically connected to the caustic feed line (37) and aqueous solution of alkali metal hydroxide-containing reactant liquid can be directed from the caustic feed line (37) into the caustic mixing device (12). Method for producing an alkali metal hydroxide-containing alkali metal cyanide solid product, wherein the method comprises the following steps: (a) Preparation of a first suspension comprising a liquid phase containing aqueous alkali metal cyanide and aqueous alkali metal hydroxide, and a solid phase containing alkali metal cyanide crystals, in a suspension preparation apparatus (2); (b) Discharge of first suspension from a crystallization vessel (3) of the suspension preparation apparatus (2); (c) Producing a second suspension containing a liquid phase with aqueous alkali metal cyanide and aqueous alkali metal hydroxide, and a solid phase containing alkali metal cyanide crystals, by mixing a first suspension drawn from the crystallization vessel (3) with a reactant liquid containing aqueous alkali metal hydroxide, wherein the alkali metal hydroxide weight fraction of the reactant liquid is higher than the alkali metal hydroxide weight fraction of the liquid phase of the first suspension; (d) Introducing the second suspension into a solid-liquid separation apparatus (18) and separating the second suspension into an alkali metal cyanide solid product and a third suspension containing a liquid phase with aqueous alkali metal cyanide and aqueous alkali metal hydroxide, and a solid phase containing alkali metal cyanide crystals; (e) Removal of alkali metal cyanide solid product from the solid-liquid separation device (18); (f) Discharge of third suspension from the solid-liquid separation device (18). Method according to claim 9, characterized in that third suspension is directed from the solid-liquid separation device (18) into the crystallization vessel (3) of the suspension production device (2). Method according to claim 10, characterized in that an operating pressure differential is maintained between the solid-liquid separation device (18) and the crystallization vessel (3) while third suspension is directed from the solid-liquid separation device (18) into the crystallization vessel (3). Method according to any one of claims 9 to 11, characterized in that • the first suspension is passed into a reaction loop (26), • in reaction loop (26) a fourth suspension is produced by adding a liquid reactant containing hydrogen cyanide to the first suspension, which contains a liquid phase with aqueous solution of alkali metal cyanide and aqueous solution of alkali metal hydroxide as well as a solid phase with alkali metal cyanide crystals, wherein the fourth suspension has a lower weight fraction of aqueous solution of alkali metal hydroxide and a higher weight fraction of alkali metal cyanide compared to the first suspension, and • the fourth suspension is introduced into the crystallization vessel (3). Method according to any one of claims 9 to 12, characterized in that a reactant liquid containing aqueous solution of alkali metal hydroxide is introduced into the crystallization vessel (3) via an alkali supply line (37). Method according to claim 13, characterized in that aqueous solution The reactant liquid containing alkali metal hydroxide is removed from the alkali supply line (37) and added to the first suspension in step (c).

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